The present invention relates to a radio system for remote control operation in a stationary vehicle by transmitting high frequency radio signals in the meter wavelength band with a radio transmitter located outside the vehicle, which transmits radio signals in a predetermined radio channel designed only for a receiver located in the vehicle.
Radio systems of this type are known from every day applications, such as vehicular telemetry devices for operation of the vehicle central lock mechanism, heating and similar remote control functions. A high degree of reliability is required for vehicle central lock mechanisms, in which the vehicle is opened with a coded signal from a hand-held transmitter acting as an electronic key. This type of device has two main problem areas. First, the transmitter power should be kept as small as possible because of the miniature battery provided in the small hand transmitter. Inside of a predetermined maximum operating radius of a few tens of meters around the vehicle it should be reliably opened by initiating a radio signal from the hand transmitter device designed for the receiver in the vehicle. In order to prevent undesired opening of the vehicle by erroneous operation of the hand transmitter outside of this predetermined maximum operating radius, it is desirable that the inner radius of the area, in which response of the receiver can be practically prevented, is not substantially larger than the predetermined maximum operating radius for the transmitter device. This condition can be fulfilled only very poorly with antenna apparatus used in practice with a single antenna because of multi-path propagation and difficulties with polarization dependent and omnidirectional antennas. Furthermore the transmission polarizations are also not well defined or definite because of the indefinite or arbitrary manipulation of the transmitter acting as key, so that in principle omnidirectionality for arbitrary polarization should be provided.
The area within the predetermined maximum operating radius 18 and the inner radius 19 of the area in which erroneous operation can be prevented are illustrated in FIG. 10.
Integrated vehicle antennas have especially pronounced indentations in their directional characteristic diagram because of the influence of the vehicle chassis. In order to achieve reliable response within the range of this predetermined maximum operating radius in the azimuthal direction at these indentations, it is necessary to select a transmitter power for which the sensitivity attainable in the receiver is sufficiently large, so that an undesirably large range for receiver response is obtained in other azimuthal directions at pronounced maxima in the directional characteristic diagram.